transmission shifts from 2nd to 3rd hard

i have a chevy k 1500 {1994} it has a 4.3 vortec engine
I had the transmission rebuilt because the transmission places that i brought it to {ammco , barry`s transmission, } told me ther was a lot of metal in the pan .
I changed the fluid in my transmission since new, every 30,000 miles i never saw any major metal and i have over a 100,000 miles on the trans.
I went to an old transmission guy and he says it will do that when the vehicle gets old {i pulled the transfer case out and brought it to him and he tore it apart and said there was nothing wrong and charged me a 100.00
It clunks from 2nd to 3rd and back
I was wondering if a new ecm chip might take up some of the slack {because that`s what it feels like}
I was also wondering if the vehicle speed sensor might be at fault and or the throttle kick down cable or the throttle position sensor
I know the vehicle is old but everything is tight {ujoints}

You sure the u-joints are fine?

You really can't test them by hand.

Ever change the tranny filter?

You sure the u-joints are fine?

You really can't test them by hand.

Ever change the tranny filter?
when they put the rebuilt trans in they replaced the ujoints and i told them it was still clunking they put a mechanic in the passenger seat and i drove the truck and he heard the sound also but could not figure it out so they gave me a life time warranty saying the trans would outlast my truck
another thing that i find ,is if it is wet a lot , {rain} it does not clunk until warmed up
yes i replaced the filter every 30,000 miles {you have to drop the pan to change the fluid so i replaced at the same time
thanks for the reply